1. Field of the Invention
The present invention relates generally to RF transmission lines, and particularly to REF terminations for said RF transmission lines.
2. Technical Background
In reference to FIG. 1, a block diagram of an RF device 1 connected to termination impedance 3 is shown. The termination impedance 3 is employed to prevent an RF signal from being reflected (from the end of the transmission line) back into device 1. As those of ordinary skill in the art will appreciate, signal reflection occurs when a signal propagates down a transmission line and encounters an impedance mismatch. (The amount of reflected energy depends on the impedance mismatch). When signals are reflected back into the device 1, the 1device performance can be degraded, and worse yet, the device itself can be damaged.
By way of another example, FIG. 2 is a diagrammatic depiction of an RF directional coupler 1′ that is often employed in RF applications. The directional coupler 1′ includes a first transmission line 5-1 disposed in parallel with a second transmission line 5-2. The coupler 1′ may be configured as, e.g., a quarter wavelength (λ/4) coupler. The first transmission line 5-1 includes port 2-1 and 2-3, whereas the second transmission line 5-2 includes port 2-2 and 24. The port 2-3 is connected to a termination resistor 3, which in turn, is coupled to ground potential. As before, if the device 1′ is not terminated properly, signal energy can be reflected back into the directional coupler 1′ and degrade its performance (such as return loss).
Now that some context has been provided, it should be noted that some of the issues impacting the design and manufacture of termination devices are related to device size, ease and simplicity of manufacture, power handling capability, bandwidth and impedance matching considerations.
In one approach that has been considered, high power RF terminations 3 can be produced using a thick film process that deposits substantially rectangular resistive patches onto a dielectric layer. When the subsequent termination device 3 is in use, it is typically mounted on a heat sink. The resistive patches are configured to convert the RF energy to thermal energy (i.e., I2R losses) so that the dielectric layer conducts the heat to the underlying heat sink. The power handling of the RF termination is proportional to the area of the resistive patch. Thus, those skilled in the art will appreciate that higher power handling can be achieved by increasing the size of the termination 3.
This approach, however, has drawbacks. For example, relatively large resistive patches are typically commensurate with relatively large parasitic capacitances that limit the high frequency performance to about 1 GHz. In order to improve the high frequency performance, designers typically employ additional tuning components (i.e., inductance) to substantially eliminate the parasitic capacitance at the resonant frequency. While additional tuning components may be employed to substantially eliminate the parasitic capacitance, the designer must also take into account the fundamental tradeoff between bandwidth and power handling.
In another approach that has been considered, an RF termination element may be implemented using a relatively long lossy transmission line that is disposed on a dielectric layer. Referring to FIG. 3, therefore, a detail schematic diagram of a termination element realized by a lossy transmission line is shown. The characteristic impedance of the transmission line is Zo, which is equal to the system impedance (Zs), which is typically about 50 Ohms. The lossy transmission line 3-1 is configured to have a length (L) so that the wave travels a distance that is substantially equal to two times of the physical line length (2*L). The end of the lossy transmission line can be either connected to ground or left as an open circuit.
In operation, an incident signal wave propagates to the end of the transmission line and then is reflected back toward the signal source. However, as the reflected RF signal propagates toward the signal source, the lossy T-line termination causes the RF energy to be converted into thermal energy (I2R losses); and thus, the reflected signal decays due to the thermal losses. By properly selecting the length of the lossy transmission line, the reflection is attenuated to a negligible level when it returns to the RF device port (i.e., the signal source or input) because the reflected RF power has been converted to heat. This approach has very good high frequency response and there is no confliction between the bandwidth and power capability.
This approach also has drawbacks: because of practical limitations, the termination depicted in FIG. 3 is best realized using stripline technology. As those skilled in the art will appreciate, a stripline device requires a relatively complicated multilayer structure that includes interlayer vias and the like. Accordingly, the manufacture of a termination device of this type requires a more complicated and expensive process than what is typically employed in a standard thick film process.
To be specific, the lossy device 3 shown in FIG. 3 is typically manufactured using a co-fired ceramic build process that includes four green ceramic dielectric layers. The lossy transmission line 3-1 is typically implemented in two parts; i.e., a circuit trace metallization process prints the transmission line on two respective dielectric layers. The dielectric layers with the trace layers must be stacked-up in the correct order. In other words, each metal trace layer is unique within the stack up and has a unique beginning and a unique end. Moreover, each trace may have a different trace width and or distance to ground because of the electrical RF design requirements. Because two traces are employed to implement lossy line 3-1, vias are required to connect to the layer above to the layer below. As a result, the required via holes must be “punched” into the respective layers and filled with a conductive material. (The lossy device 3 can also be made with a bare minimum of two dielectric layers instead four. In this case, the circuit trace is sandwiched between the two layers, with the outer surfaces of the top and bottom layer including ground metallization. Even so, vias are still required because the center trace must be connected to the exterior of the device, and the top and bottom ground layers must be interconnected). After these steps are completed, the “stack-up” is fired to cure (harden) the green ceramic and conductive material in the vias and trace layers. The exterior surfaces are then metalized with nickel plating. (The plating could also employ silver, gold, tungsten, or conductive non-metallic materials such as graphite).
What is needed therefore is a termination device that offers performance similar to a lossy transmission line while overcoming its drawbacks. For example, a lossy termination device is needed that can be produced using standard thick film processes. As such, a lossy termination device is needed that requires a transmission line that has higher impedance and a smaller line length. What is further needed is a termination device that can be implemented using a microstrip structure rather than a more complicated and expensive stripline structure.